The use of plasma technology to modify the surface of polymeric structures such as hollow fibers in order to produce semipermeable membranes has been known in the art, see, for example, U.S. Pat. Nos. 4,046,843 and 4,147,745 hereby incorporated herein by reference. The surface modification of these structures, however, does not enhance the bulk porosity of the resultant membrane and therefore the membrane cannot be utilized for many specific service applications for which it would otherwise prove useful.